Write once optical recording medium and recording method and write once optical recording medium

ABSTRACT

To obtain a write-once high density optical recording medium capable of a high speed recording, and to effectively form space between recording marks.  
     The recording layer  18  of the high speed write-once optical recording medium  10  is formed by laminating a first sub recording layer  18 A and a second sub recording layer  18 B each containing a metal as its main component. When the recording layer (laminated recording layer)  18  is irradiated with a laser beam, the main component metals contained in the first and second sub recording layers  18 A,  18 B diffuse and are mixed together by means of the irradiation, thereby forming recording marks whose reflectance have been irreversibly changed due to such mixing. At this time, after an irradiation using a laser beam for forming recording marks, a power level P1 lower than a read power is maintained for a predetermined time T1.

TECHNICAL FIELD

The present invention relates to a write-once optical recording mediumand a recording method for the write-once optical recording medium.

BACKGROUND ART

Now, in the market of optical recording medium there have beendistributed rewritable optical recording media and so-called write-onceoptical recording media which are not rewritable. A rewritable opticalrecording medium, as meant by the word “rewritable,” allows data to bewritten therein again and again, so that it is possible for the sameoptical recording medium to be used repeatedly in recording onlyrequired data. On the other hand, a write-once optical recording mediumis not rewritable and thus characterized by a feature that “data willnever be altered,” thereby making itself useful in data distribution,storage, or backup.

Conventionally, as a recording structure of a write-once opticalrecording medium, it has been suggested that organic dyes is applied toa substrate. However, such a recording structure has been foundinsufficient in its recording sensitivity when performing a high speedrecording. Furthermore, if the wavelength of a laser beam is made shortin order to increase a recording density, there will be a problem thatit is difficult to synthesize dyes which can be used with laser beamshaving wavelengths equal to or shorter than that of a blue light.

Although there have been several suggestions (for example, JapanesePatent Publication No. 1992-838, etc.) that recording layer can beformed by an inorganic material, none of conventional structures issuitable for high density or high speed recording, their storagereliability of recorded state is inadequate, and their playbackdurability is insufficient.

For this reason, among various high density optical recording mediausing laser beams having wavelengths equal to or shorter than that of ablue light, what has been suggested as an optical recording mediumcapable of “high speed recording” is only a rewritable (RW) opticalrecording medium formed by using a phase-change material.

Recently, in the field of optical recording media for use as amultimedia-compatible medium, there has been a demand for a higherdensity and higher speed recording. Similarly, the same demand isexisting in the field of write-once optical recording media.

With regard to a rewritable optical recording medium, it is necessary tostrictly control several time-related factors such as cooling speed,with the recording strategy becoming more complex because of high speedand high density recording. On the other hand, a write-once opticalrecording medium has a recording strategy which was not as complex as arewritable optical recording medium. However, in view of a furtherhigher density and further high speed recording, what has been clearlyunderstood is that it is impossible to obtain sufficient characteristicsby a recording strategy using a conventional write-once opticalrecording medium. Here, the recording strategy means a power controlpattern of a recording laser beam. Generally speaking, a recording laserbeam (especially when a recording is performed on an optical recordingmedium using a phase-change material) is not continuously irradiatedcorresponding to the length of recording mark. In contrast, as describedin Japanese Patent Laid-Open Publication No. 1997-7176, for the purposeof controlling the shape of recording mark, a general practice is toirradiate a laser beam formed by a pulse train consisting of a pluralityof pulses, with the width of each pulse in the pulse train strictlycontrolled. At this time, detailed arrangement of pulse dividing isusually designated to as recording strategy.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished in order to solve theabove-described problems existing in conventional recording media. It isan object of the invention to provide a write-once optical recordingmedium capable of maintaining spaces between recording marks at cleannon-recorded portions, and a recording method for the write-once opticalrecording medium.

The above-mentioned object can be achieved by inventions indicated inthe following (1) to (9).

(1) 1. A recording method for a write-once optical recording medium,characterized in that: a laminated recording layer containing at leasttwo sub recording layers is provided, each sub recording layercontaining one kind of metal as its main component; a laser beam isirradiated onto the laminated recording layer, so that the maincomponent metals contained in the respective sub recording layers arediffused and mixed; the laminated recording layer is changed to a singlelayer through the mixing to be recordable a recording mark whosereflectance is irreversibly changed; and after a laser beam having apredetermined write power Pw for forming the recording mark isirradiated, another laser beam having a power level P1 lower than a readpower Pr set for reading the recording mark is irradiated for apredetermined time.

(2). The recording method for a write-once optical recording medium,according to (1), wherein a laser beam having a power level P2 higherthan the read power Pr and lower than the write power Pw is irradiated,before the followed by performing another irradiation using thepredetermined write power Pw for forming the recording mark.

(3). The recording method for a write-once optical recording mediumaccording to (1), wherein after the laser beam having the power level P1lower than the read power Pr is irradiated, a laser beam having a powerlevel P2 higher than the read power Pr and lower than the write power Pwis further irradiated.

(4). The recording method for a write-once optical recording mediumaccording to any one of (1) to (3), wherein at least a length of ashortest recording mark among the recording marks is less than 0.35times the spot diameter of the laser beam.

(5). The recording method for a write-once optical recording mediumaccording to any one of (1) to (4), wherein at least a width of theshortest recording mark among recording marks is 0.7 times or more thelength of the shortest mark.

(6). The recording method for a write-once optical recording mediumaccording to any one of (1) to (5), wherein a wavelength of the laserbeam is set within a range of 200 to 450 nm.

(7). A write-once optical recording medium, comprising

-   -   a laminated recording layer containing at least two sub        recording layers each containing one kind of metal as its main        component, wherein the laminated recording layer with is        irradiated with a laser beam having a predetermined recording        strategy so that the main component metals contained in the        respective sub recording layers are diffused and mixed together        to form a single layer, in order to be recordable recording        marks whose reflectance are irreversibly changed, and the        recording strategy comprises a configuration such that a laser        beam having a write power Pw is irradiated for forming a certain        recording mark, and then another laser beam having a power level        P1 lower than a read power Pr set for reading recording marks is        irradiated for a predetermined time.

(8). The write-once optical recording medium according to (7), whereinthe laminated recording layer comprises two sub recording layers, one ofwhich contains as its main component metal one element selected from thegroup comprising Al, Ag, Au, and Cu.

(9). A write-once optical recording medium characterized in that: themedium is to be irradiated with a laser beam having a wavelength of 450nm or shorter in accordance with a predetermined recording strategy at arecording transfer rate of 35 Mbps or higher, thereby forming arecording mark, wherein the predetermined recording strategy comprises aconfiguration such that before a laser beam having a write power Pw isirradiated, a laser beam having a power level P2 higher than a readpower Pr set for reading a recording mark and lower than a write powerPw for forming a certain recording mark is irradiated.

(10). The write-once optical recording medium according to any one of(7) to (9), wherein at least a length of a shortest recording mark amongthe recording marks is less than 0.35 times the spot diameter of thelaser beam.

The technical summery of the present invention will be explained asfollows.

An optical recording medium to which the present invention is appliedhas a laminated recording layer of a structure basically formed bylaminating at least two sub recording layers. Each sub recording layercontains one kind of metal as its main component. Once the laminatedrecording layer is irradiated with a laser beam, the main componentmetals contained in the respective sub recording layers are diffused andmixed together in an irradiated area.

Recording marks formed by the diffusion and mixing reaction of therespective main component metals are extremely stable due to a singlelayer structure, and its reflectance has been irreversibly changed.Thus, the change of the reflectance can be caught as information, andsuch a recording mark after recording is not likely to change, even ifthe recording medium is stored or played-back in an environment of anelevated temperature.

The formation of the recording mark can be effected only by a heatamount control which can ensure progression of a predetermined diffusionand mixing of the main component metals in the sub recording layersbasically by means of an irradiation using a laser beam. Accordingly,even in relation to a high speed recording, it is still possible tocarry out a flexible treatment by adjusting the heat amount control.

Consequently, although at a low cost, it is possible to perform a highdensity recording at a high speed, thereby obtaining an opticalrecording medium suitable for use as a multimedia-compatible medium.

Particularly, in the present invention, after a laser beam having apredetermined write power Pw for forming the recording mark isirradiated, another laser beam having a power level P1 lower than theread power Pr is irradiated. As a result, it becomes possible to preventsome problems that are likely to occur when performing a high densityrecording at a high speed, particularly to effectively prevent a problemthat a heat of a laser beam excessively remains at the terminal of along recording mark, and such a remaining heat makes it impossible toclearly form a space (a portion in which any recording mark is notformed) until a next recording mark.

Moreover, by forming an arrangement of performing an irradiation using alaser beam having a power P2 higher than a read power and lower than awrite power, it is possible to compensate for a recording sensitivitydecrease occurring during a high speed recording. In particular, as willbe described later, it is possible to solve the recording densitydecrease occurring when shortening the pulse length at the time offorming the shortest recording mark in order to achieve a high densityrecording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross sectional view schematically showing ahigh-speed write-once optical recording medium according to anembodiment of the present invention;

FIG. 2 is a pulse waveform graph showing an example of a recordingstrategy at the time recording marks are being formed on theabove-mentioned high-speed write-once optical recording medium; and

FIG. 3 is an explanatory view showing a comparison between aconventional method and a method of forming the shortest recording markin the above-mentioned embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in detail withreference to the accompanying drawings.

As shown in FIG. 1, a high-speed high-density write-once opticalrecording medium (hereinafter referred to as optical recording medium)10 to which the present invention is applied is formed by successivelyforming, on a support substrate 12, a reflection layer 14, a seconddielectric layer 16, a recording layer (laminated recording layer) 18, afirst dielectric layer 20, and a light transmitting cover layer 22. Ablue laser beam having a wavelength of 405 nm, for example, is emittedby a recording laser beam source 24 to irradiate the above-mentionedrecording layer 18 through the light transmitting cover layer 22,thereby changing the reflectance of an irradiated area and using theirradiated area as a recording mark.

Preferably, at least one of the sub recording layers forming therecording layer 18 contains, as its main component metal, at least oneof high reflectance metals including Al, Ag, Au, and Cu etc. In detail,it is allowed to use Al—Sb, Al—Ca, Al—Ce, Al—La, Al—Se, Au—Ce, Au—La,Au—Si, Au—Ge, Si—Cu, Ge—Cu, etc.

Particularly, since Al and Ag exhibit high reflectivity even with alaser beam having a wavelength equal to or shorter than that of bluelight, it is easy to adjust the reflectance by changing the thickness ofsub recording layers. Further, it is possible to set the reflectance ofa non-recorded portion at a value higher than the reflectance of aportion in which recording mark has been formed, thus providing a morepreferable result.

Moreover, it is more preferable to form an inter-metallic compoundhaving a melting point higher than either any one of the sub recordinglayers or any one of the above main component metals.

When a blue laser beam is used as a recording beam to irradiate therecording layer 18, the main component metals contained in first andsecond sub recording layers 18A and 18B will diffuse and are thus mixedtogether in an irradiated area, thereby producing a reaction productwhich is a single layer formed by such a mixing. Further, such areaction product changes the reflectance of the irradiated area, thusforming recording mark which can be recognized. In this way, since theabove reaction involving diffusion of the two main component metals aswell as the mixing thereof is irreversible, the recording layer 18 canbe used to perform write-once optical recording.

The thickness of the recording layer 18, which is an entire thicknessincluding the first and second recording layers 18A and 18B, is set tobe 3 to 50 nm, preferably 5 to 20 nm. Qualitatively, if the recordinglayer 18 (including sub recording layers 18A and 18B) is too thin, itwill be difficult to ensure an adequate reflectance difference in arecording mark between a state before recording and a state afterrecording. On the other hand, if the recording layer 18 is too thick, aheat capacity will become large, hence reducing a recording sensitivity.

Accordingly, the thickness of the sub recording layers 18A and 18Bshould be appropriately set such that a high thermal stability can beobtained and recording mark having large reflectance difference can beformed. For example, when a sub-recording layer containing Al as itsmain component is combined with a sub-recording layer containing Sb asits main component, it is allowed to consider that an inter-metalliccompound will be formed by combining Al with Sb at a ratio of 1:1.Therefore, it is preferable to set the thickness of each sub-recordinglayer in a manner such that a ratio (atomic ratio) of Al to Sb in therecording layer 18 will not deviate greatly from 1:1.

The first and second dielectric layers 20, 16 may be formed by variousdielectric materials including an oxide, a sulfide, a nitride, afluoride, and a carbide, or mixtures thereof. In the present embodiment,the first and second dielectric layers 20, 16 are all formed throughsputtering using ZnS—SiO₂ target (ZnS: 80 mol %, SiO₂: 20 mol %).

The first dielectric layer 20 has a thickness of 5 to 200 nm and isprovided to cooperate with the second dielectric layer 16 so that therecording layer 18 can be interposed between the two dielectric layers.Here, the second dielectric layer 16 has a thickness of 5 to 200 nm andis provided on the reflection layer 14.

The first and second dielectric layers 20, 16 also serve to protect therecording layer 18 from vapor or other gases. Further, by adjusting thethickness of these dielectric layers, it is possible to interfere thelaser beam by the adjusted thickness, and to adjust the reflectance ofnon-recorded portion of the recording layer 18, as well as to furtherincrease a reflectance difference between a state before recording and astate after recording.

The support substrate 12 comprises a polycarbonate plate having athickness of 1.1 mm.

Moreover, the reflection layer 14 is for example a silver alloy layerhaving a thickness of 10-200 nm, which is formed on the supportsubstrate by means of sputtering or the like. When viewed from theincident side of the laser beam, such reflection layer 14 is locatedinwardly of the recording layer 18. By providing a returning beam to therecording layer 18, it is possible to increase the reflectancedifference between a state before recording and a state after recording,as well as to improve a recording sensitivity because of the reflectionlayer 14. Specifically, the reflection layer 14 is formed by a metal(including metalloid) film or a dielectric multi-layer film. In thepresent embodiment, the reflection layer 14 is formed by an alloy whichis AgPdCu, having a thickness of 100 nm and containing silver as itsmain component. However, such a reflection layer 14 is not absolutelyindispensable.

The light transmitting cover layer 22 is formed by spin-coating thefirst dielectric layer 20, or formed by bonding thereon a sheet-likematerial shaped in advance, such as an ultraviolet setting resin layeror a polycarbonate sheet. The thickness of the light transmitting coverlayer 22, or an entire thickness including the cover layer 22 and thefirst dielectric layer 20, should be selected such that when thenumerical aperture (NA) of an objective lens 26 at the time ofirradiating the recording layer 18 with a blue laser beam having awavelength of 405 nm is set at 0.85, the blue laser beam can beconverged on the recording layer 18. In the present embodiment, such anentire thickness is set at 100 μm.

Specifically, the optical recording medium 10 of the present embodimentis characterized in that if the recording layer 18 comprises a first subrecording layer 18A containing Al as its main component and a second subrecording layer 18B containing Sb as its main component, the thermalstability (namely, thermal stability after recording) of reactionproduct in recording mark formed on the recording layer 18 will behigher than a thermal stability (namely, thermal stability beforerecording) when the first and second sub recording layers 18A, 18B aresimply laminated on non-recorded portion.

In more detail, during an irradiation using a laser beam, the maincomponent metals contained in the first and second sub recording layers18A and 18B will diffuse respectively and be mixed together, therebyforming an inter-metallic compound. On the other hand, even if aninter-metallic compound is not formed, it is allowed to consider that atleast the main component metals will be combined with each other, thusforming a mixture. Thus, since a reaction product formed by such amixing can irreversibly alter the reflectance of an irradiated area, itis allowed to make use of the reflectance variation as a recording mark.

The melting point of Al is 660° C. and the melting point of Sb is 631°C. Namely, both of them have melting points considerably higher than500° C. and have sufficient thermal stability when existing as simplesubstance, and can be fused upon being irradiated by a laser beam.Further, by means of a reaction between Sb and Al, it is possible toform a stable inter-metallic compound AlSb (melting point: 1060° C.)whose melting point is sufficiently higher than that of each respectivesimple substance and whose crystal structure will not change either at alow temperature or at an elevated temperature. On the other hand, aninter-metallic compound such as AlSb is not necessary to be in crystalgrowth, and is capable of recording even in a microcrystal state whichcan not be detected by an electron ray diffraction.

Upon catching the above facts as phenomenon, when the recording layer 18is irradiated with a laser beam having a write power Pw capable offorming recording mark, the following facts can be confirmed: A) in someareas of the recording layer 18 in which mixing has not occurred, suchmixing will thus occur and reflectance will be changed (recordingbecomes possible), B) in other areas in which recording marks havealready been formed, reflectance will not be changed even if these areasare irradiated by the recording beam, thereby providing idealcharacteristics as write-once type.

Therefore, the optical recording medium 10 is such that its recordingmarks comprising the above-mentioned reaction product are not easy tochange and they are stable even if the recording medium is stored in ahigh temperature environment after recording. Further, the recordingmarks are not likely to change even if the recording medium is subjectto a continuous playback, thereby exhibiting an excellent playbackdurability. Moreover, even if the reflectance after formation ofrecording marks is set to be low and light absorption rate afterformation of recording marks is set to be high, since the thermalstability of recording marks is high, the recording medium will not getdeteriorated by an irradiation of a playback laser beam.

Moreover, since the thermal stability of recording marks is high, aphenomenon of accidentally erasing the recording marks in an adjacenttrack (cross erase) at the time of recording can be substantiallyprevented. In this way, since it is possible to make narrow the pitch ofrecording tracks, such an optical recording medium can be effectivelyused in high density recording (this will be described later).

FIG. 2 shows examples of recording strategies for forming recordingmarks. Particularly, FIG. 2 shows examples of forming relatively longrecording marks R. However, the examples shown in FIG. 2 haveexaggerated a qualitative concept, so that the size, number andmaintaining time of the respective pulses will not be necessarily thesame as actually measured.

FIG. 2(A) shows one of the most fundamental examples of the presentembodiment. In this example, as a strategy at the time of recording, aswitching arrangement having a write power Pw and a bias power Pb isused as a basic arrangement. Then, after a laser beam having apredetermined write power Pw (FP and three MPs shown in the example) hasirradiated for forming recording marks R, a laser beam having a powerlevel P1 lower than a read power (a laser power set for readingrecording marks R) Pr is allowed to irradiate for a predetermined timeT1, thus forming an additional arrangement.

When a high density recording is performed and a recording transfer rateis high, a heat of laser beam will excessively remain near the terminalsof particularly long recording marks R. Because of such remaining heat,the rear portions of the recording marks will form into tail-like areas,and the edge of rear portion of each mark is likely to form into aslackened portion. At this time, there will be a problem that markportion enters a space until a next recording mark and thus the spacecan not be clearly formed. Especially, when mark edge recording isperformed, since a boundary between a mark and a space is not clear, ajitter value of each signal will get deteriorated and an error is likelyto occur.

On the other hand, since a power level P1 lower than a read power Pr ismaintained for a predetermined time T1 in the last portion of recordingmark formation, it is possible to prevent the occurrence of the aboveproblem.

By using such a strategy, it is possible to obtain a write-once opticalrecording medium whose shortest mark (space) length is 0.35 with respectto a spot diameter (provided as a laser wavelength λ/lens numericalaperture NA), thus allowing a high density recording. In detail, whenthe laser wavelength is 405 nm and the lens numerical aperture is 0.85,the shortest mark length will be less than 167 nm.

An appropriate range of the maintaining time T1 of the power level P1 is0.2 to 1.0 times the passing time of the shortest mark (space) length,preferably 0.3 to 0.8 times the passing time. However, it is alsoallowable to make the bias power Pb equal to the power level P1, and tomake the maintaining time T1 equal to or longer than the length of thelongest space.

Specifically, an appropriate range of the power level P1 is from thecritical value of track trace to 0.4 mW. Since the critical value oftrack trace is 0.08 mW at present time, it is more preferable that 0.1mW to 0.3 mW becomes an optimum range at present time. In the future, ifthe trace capability of hard system has been improved, it is allowed toreduce the lower limit correspondingly.

FIG. 2(B) is based on the arrangement of FIG. 2(A). In order to form theabove-mentioned recording marks, prior to an irradiation with apredetermined write power Pw, another irradiation is performed by usinga laser beam having a power level P2 which is higher than the read powerPr and lower than the write power Pw. When recording is performed at ahigh speed, with regard to a laser beam rise time and a recordingsensitivity, it is effective to set the laser power P2 higher than theread power Pr and to perform preheating. In particular, as in thepresent embodiment, when the power level P1 lower than the read power Pris maintained after the formation of a recording mark, it is possible tosatisfactorily form a rise portion of a next recording mark.

Here, although the power level P2 is higher than the read power Pr, itis set at a sufficiently lower value than the write power Pw. Thepurpose of such setting is to prevent the laser beam irradiation of thepower level P2 from producing non-intended recording marks innon-recorded portions. In more detail, a range of 1.1 Pr to 0.4 Pw issuitable, with 1.2 Pr to 0.3 Pw being the best range.

Although the maintaining time T2 of the power level P2 is preferable tobe set at 0.2 to 1.0 times the passing time of the shortest mark length,it is also allowed to be used as the power level of P2 immediately afterirradiation of P1 (FIG. 2(D)).

As shown in FIG. 2(C) as well as in FIG. 2(B), it is also possible tohave the bias power Pb at the time of forming recording marks to becoincident with the power level P1 (Pb P1).

FIG. 2(D) shows that when in FIG. 2(C) the bias power Pb at the time offorming recording marks is at a value slightly lower than the powerlevel P2 (Pb is nearly equal to P2). In this way, it is possible torectify the middle portion of each recording mark into a shape close toa straight line. Particularly, by setting the power level of Pb at avalue equal to or slightly lower than P2, it is possible to set thewrite power at a low value, thereby compensating for a recordingsensitivity decrease which is unfavorable for a high-speed recording.

In any of the above examples (A) to (D), with the same pattern ofrecording strategy being in use, it is substantially possible to handlea variable-speed recording simply by changing the frequency and thewrite power Pw. At this time, if the values of bias power Pb or powerlevels P1 and P2 are changed and set according to a recording transferrate, it is possible to perform a further precise fine adjustment onrecording marks R such that they will not become tear drop type orreverse tear drop type.

Particularly, in the case where recording is performed on a driveapparatus capable of performing a test-write on the recording medium 10prior to recording information, during the test-writing, if the value ofthe bias power Pb, the maintaining time T1 of the power level P1immediately after the formation of recording mark, and the maintainingtime T2 of the power level P2 are adjusted, it is possible to obtaingood recording characteristics at any recording transfer rate among 35Mbps to 100 Mbps. However, it is also possible for time T1 and time T2to be changed independently.

Incidentally, in the present embodiment, as shown in FIG. 3, the shapeof the shortest recording mark R1 is not set to be an elliptical marklike a conventional recording mark Ro, but has a shape close to a truecircle. Here, FIG. 3 shows as reference some examples indicating thesizes of recording marks of DVD-Rs.

In the case of a write-once optical recording medium using conventionaldyes, since it is necessary to form the recording mark Ro accompanyingdeformation of groove section, it was not allowed to record therecording mark Ro beyond groove so as to avoid signal distortion. Forthis reason, in order to obtain playback signals having a certaindimension under a condition in which the width (in radial direction) ofthe recording mark Ro is limited, it was absolutely necessary to formthe elliptical shape having a corresponding length in thecircumferential direction. However, with regard to the optical recordingmedium 10, as described above, since it is possible to avoid groovesection deformation when forming recording mark R1, and since the resultis signal amplification even under a condition in which recording isperformed beyond group section, no trouble would occur even if recordinghas been done beyond groove section.

For this reason, even in the case where track pitch is narrowed andgroup width is reduced for high density recording, it is possible forthe shortest mark to be formed into a shape close to a true circle, andfor its width to be 0.7 times or more, preferably 0.8 times or more thelength of the shortest mark, which was only 0.5 (times) the length ofthe shortest mark in prior art.

In order to make the shortest recording mark to be close to a truecircle, the write power is increased and the pulse width is reduced. Inmore detail, although an irradiation was usually conducted with a laserpulse having the write power Pw, in a range of 0.3 to 0.7 times thepassing time of the shortest mark length, the present embodiment is suchthat the laser irradiation is conducted in a range of 0.05 to 0.3 timesthe passing time of the shortest mark length. In this way, as shown inFIG. 3, it is possible to obtain relatively large playback signal evenif the lengths in the circumferential direction are the same, therebyimproving CNR (carrier to noise ratio). Further, if the magnitude ofplayback signal is only required to be the same as prior art, it ispossible to correspondingly realize a recording with a higher density.

Incidentally, when the write power is increased in order to make theshortest recording mark close to a true circle in this way, theabove-described arrangement “an irradiation is conducted using a laserbeam having a power level P1 set at a lower value than the read power Prset at the time of reading recording mark” is effectively contributiveto a clear formation of space. Further, another arrangement “prior to anirradiation using a predetermined write power Pw for forming recordingmarks, another irradiation is performed using a laser beam having apower level P2 higher the read power Pr but lower than theabove-mentioned write power Pw” is effectively contributive to acompensation for a power shortage caused by a shortened pulse width. Ofcourse, these arrangements may be adopted simultaneously.

The operation of a recording method for the optical recording medium 10formed according to the present embodiment will be described as follows.

The formation of recording marks on the recording layer (laminatedrecording layer) 18 formed according to the present embodiment can beeffected sufficiently only by performing a heat amount control forcarrying out the predetermined diffusion and mixing among main componentmetals in sub recording layers basically by means of laser beamirradiation, thereby making it possible to perform a high speedrecording with ease by adjusting the heat amount control. Further, sincethe laminated recording layer is formed by using metal materials havinga high thermal conductivity, it is not necessary to worry about aninfluence from a large heat interference generated in the opticalrecording medium containing dye materials. Moreover, since it ispossible to form each recording mark close to a true circle, it ispossible to ensure high density recording.

Furthermore, by using the recording strategies shown in FIGS. 2(A) to2(D), it is possible to clearly maintain an edge portion of mark/spaceand to compensate for a recording sensitivity decrease during a highspeed recording, which would otherwise become a problem in realizing ahigh density during a high speed recording.

In this way, it becomes possible to realize a high speed and highdensity recording which was difficult with a write-once opticalrecording medium.

In particular, after an irradiation using a write power Pw, since thearrangement is to perform an irradiation using a laser beam having apower level P1 lower than a read power Pr, it is possible to minimize aninfluence from a remaining heat (even when a long recording mark hasbeen formed), and to leave a space until a next recording mark as aclean non-recorded portion. Furthermore, even in the case where a writepower Pw has been set large in order to form the shortest recording markin a shape close to a true circle, it is still possible to effectivelyeliminate the influence from the remaining heat and to leave a spaceuntil a next recording mark as a clean non-recorded portion.

Although the recording layer 18 comprises the first and second subrecording layers 18A and 18B, it is also possible for the recordinglayer to comprise three or more sub recording layers, and allow any oneof the sub recording layers to be located on the incident light side,provided that the recording layer comprises at least two sub recordinglayers. Further, the first and second sub recording layers 18A and 18Bforming the recording layer 18 are allowed to contain only maincomponent metals, or other additional elements besides main componentmetals. Moreover, although the above embodiment showed that therecording layer 18 is formed such that the first sub recording layer 18Aand the second sub recording layer 18B are in direct contact with eachother, it is also allowed to interpose an intervening layer containingother elements as its main component between the two sub recordinglayers.

The above embodiment shows that when information is recorded in detailin the optical recording medium 10, the wavelength of the laser beam isset at a blue wavelength of 405 nm. However, it is also possible for thewavelength to be set equal to or shorter than such a wavelength. Rather,it is allowed to say that only when a high speed recording is conductedunder such a condition, it is possible to make full use of theadvantages of the present invention. On the other hand, it has also beenconfirmed that even when using a laser beam having a red wavelengthlevel which is longer than a blue wavelength level, it is still possibleto obtain the predetermined advantages of the present invention (indetail, using a laser beam having a wavelength range of 200 nm to 700nm). Therefore, for example, if it is desired to realize a further lowercost, it is possible for the present invention to be applied to a laserirradiation system using a red wavelength.

EXAMPLE 1 Example of Al/Sb

An optical recording medium was produced in accordance with anarrangement shown in FIG. 1, and an evaluation was conducted concerninga high-density and high-speed recording.

A polycarbonate substrate of 1.1 mm, having grooves whose track pitchwas 0.32 μm and groove width was 0.13 μm, was used to form the supportsubstrate 12, while the thickness of the light transmitting cover layer22 was set at 100 μm.

Other layers were produced by sputtering under the following condition.

-   -   Electrostatic layer: ZnS+SiO₂ (80:20 mol %)    -   First dielectric layer 20: 60 nm    -   Second dielectric layer 16: 105 nm    -   First sub recording layer 18A: AlCr (98:2 at. %) 4 nm    -   Second sub recording layer 18B: Sb 6 nm    -   Reflection layer 14: AgPdCu (98:1:1 at. %) 100 nm

The shortest mark (2T) was changed, random signals were recorded, andevaluation was carried out with playback Jitter value, all by using anevaluation apparatus whose laser beam has a wavelength of 405 nm andwhose objective lens group has a numerical aperture NA of 0.85, and bychanging the recording linear velocity with an equivalent for 70 Mbps((1, 7) RLL modulation manner, channel clock 132 MHz fixation, formatefficiency 80%).

A multi-pulse strategy used in recording is an (n-1) type strategy, with2T recorded by 1 pulse of FP (first pulse), 5T recorded by 3 pulses ofFP and MP (multi-pulse), thus recording by a total of 4 pulses.

A strategy having a shape shown in FIG. 2A was used, with Pr=Pb: 0.5 mWand Pw: 6.0 mW, and with the respective lengths of pulses being TFP:0.3T (0.15), TMP: 0.25T, and T1: 1T (0.5) (proportions with respect tothe shortest mark length).

At this time, P1 is set at 0.1 mW and 0.5 mW (P1=Pr comparativeexample), using the clock Jitter value of a signal to perform anevaluation, with the evaluation result shown in Table 1. RecordingJitter (%) linear velocity Longest mark P1 P1 (m/s) length (nm) 0.5 mW0.1 mW 11.4 173 7.3 6.9 10.6 161 8.1 7.5 10.3 156 9.3 8.1 10.0 152 10.68.8

It is understood that by using a recording strategy of irradiating for apredetermined time with a power level P1 which is lower than a readpower, it is possible to reduce the Jitter value and improve signalcharacteristics. Also, it is understood that this difference isparticularly remarkable when the shortest mark length is shortened andused as a high recording density.

A sample recording layer, on which 2T (shortest mark) and a singlesignal of space have been recorded at 10.6 m/s, was stripped from thesubstrate and the cover layer, followed by observing the shape ofrecording mark by means of TEM. The width of recording mark was found tohave reached 150 nm (about 0.9 times with respect to the recording marklength), with the mark being close to a true circle.

A recording strategy having a shape shown in FIG. 2(D) was used toperform an evaluation of recording marks on the same sample.

Under a condition in which P2=Pb is 1 mW, P1 is 0.1 mW, and therespective lengths of pulses and other parameters are the same as thosedescribed above, random signals were recorded at a recording linearvelocity of 10.6 m/s and then an evaluation was conducted.

It was found that an optimum write power had dropped to 5.0 mW (Jitter7.6%) and the recording sensitivity had been improved 1.0 mW from 6.0mW.

EXAMPLE 2 Example of Si/Cu

An optical recording medium was produced in accordance with thearrangement shown in FIG. 1, followed by an evaluation on a high speedand high density recording, using the same manner as Example 1.

The support substrate and light transmitting cover layer were made thesame as those in Example 1.

Other layers were produced by sputtering under the following condition.

-   -   Electrostatic layer: ZnS+SiO₂ (80:20 mol %)    -   First dielectric layer 20: 22 nm    -   Second dielectric layer 16: 28 nm    -   First sub recording layer 18A: Si 5 nm    -   Second sub recording layer 18B: Cu 6 nm    -   Reflection layer 14: AgPdCu (98:1:1 at. %) 100 nm

The shortest mark (2T) was changed, random signals were recorded, andthe playback Jitter value was evaluated, all by using an evaluationapparatus whose laser beam has a wavelength of 405 nm and whoseobjective lens group has a numerical aperture NA of 0.85, and bychanging the recording linear velocity with an equivalent for 35 Mbps((1, 7) RLL modulation manner, channel clock 66 MHz fixation, formatefficiency 80%).

A strategy having a shape shown in FIG. 2A was used, with Pr=Pb: 0.5 mWand Pw: 5.0 mW, and with the respective lengths of pulses being TFP:0.3T (0.15), TMP: 0.25T, and T1: 1T (0.5) (proportions with respect tothe shortest mark length).

Similar to Example 1, P1 is set at 0.1 mW and 0.5 mW (P1=Pr comparativeexample), and using the clock Jitter value of a signal an evaluation wasperformed with the evaluation result shown in Table 2. TABLE 2 RecordingJitter (%) linear velocity Longest mark P1 P1 (m/s) length (nm) 0.5 mW0.1 mW 5.7 173 7.9 7.4 5.3 161 8.4 7.8 5.0 152 11.1 9.1

It is understood that by using a recording strategy of irradiating for apredetermined time with a power level P1 which is lower than a readpower, it is possible to reduce the Jitter value and improve signalcharacteristics, as in Example 1. Also, it is understood that thisdifference is particularly remarkable when the shortest mark length isshortened and used as a high recording density.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to perform arecording at a high speed. Further, since it is possible to clearlymaintain an edge portion of mark/space, which becomes a problem whentrying to realize a high density in a high speed recording, it ispossible to compensate for a recording sensitivity decrease occurring ina high speed recording. Moreover, by adjusting a maintaining time havinga low power level, it is possible to better handle a high speedrecording as well as a low speed recording.

1-21. cancel
 22. A recording method for a write-once optical recordingmedium, comprising following steps of: providing a laminated recordinglayer containing at least two sub recording each containing one kind ofmetal as its main component; irradiating a laser beam onto the laminatedrecording layer to diffuse and mix the main component metals containedin the respective sub recording layers so as to be recordable recordingmarks; irradiating a laser beam having a predetermined write power Pwfor forming a certain recording mark, and irradiating another laser beamhaving a power level P1 lower than a read power Pr set for reading therecording marks for a predetermined time.
 23. The recording method for awrite-once optical recording medium, according to claim 22, whereinbefore the laser beam having the write power Pw is irradiated, a laserbeam having a power level P2 higher than the read power Pr and lowerthan the write power Pw is irradiated.
 24. The recording method for awrite-once optical recording medium according to claim 22, wherein afterthe laser beam having the power level P1 is irradiated, a laser beamhaving a power level P2 higher than the read power Pr and lower than thewrite power Pw is further irradiated.
 25. The recording method for awrite-once optical recording medium according to claim 22, wherein atleast a length of a shortest recording mark among the recording marks isless than 0.35 times the spot diameter of the laser beam.
 26. Therecording method for a write-once optical recording medium according toclaim 22, wherein at least a width of the shortest recording mark amongrecording marks is 0.7 times or more the length of the shortest mark.27. The recording method for a write-once optical recording mediumaccording to claim 22, wherein a wavelength of the laser beam is setwithin a range of 200 to 450 nm.
 28. A write-once optical recordingmedium, comprising a laminated recording layer containing at least twosub recording layers each containing one kind of metal as its maincomponent, wherein the laminated recording layer is to be irradiatedwith a laser beam having a predetermined recording strategy so that themain component metals contained in the respective sub recording layersare diffused and mixed together so as to be recordable recording marks,and the recording strategy comprises a configuration such that a laserbeam having a write power Pw is irradiated for forming a certainrecording mark, and then another laser beam having a power level P1lower than a read power Pr set for reading the recording marks isirradiated for a predetermined time.
 29. The write-once opticalrecording medium according to claim 28, wherein one of the sub recordinglayers contains as its main component metal one element selected fromthe group comprising Al, Ag, Au, and Cu.
 30. A write-once opticalrecording medium characterized in that: the medium is to be irradiatedwith a laser beam having a wavelength of 450 nm or shorter in accordancewith a predetermined recording strategy at a recording transfer rate of35 Mbps or higher, thereby forming recording marks, wherein thepredetermined recording strategy comprises a configuration such thatbefore a laser beam having a write power Pw is irradiated for forming acertain recording mark, a laser beam having a power level P2 higher thana read power Pr set for reading the recording marks and lower than thewrite power Pw is irradiated.
 31. The write-once optical recordingmedium according to claim 28, wherein at least a length of a shortestrecording mark among the recording marks is less than 0.35 times thespot diameter of the laser beam.
 32. A recording method for a write-onceoptical recording medium, comprising following steps of: irradiating alaser beam having a predetermined write power Pw for forming a certainrecording mark, and irradiating another laser beam having a power levelP1 lower than a read power Pr set for reading the recording marks for apredetermined time.
 33. The recording method for a write-once opticalrecording medium, according to claim 32, wherein before the laser beamhaving the write power Pw is irradiated, a laser beam having a powerlevel P2 higher than the read power Pr and lower than the write power Pwis irradiated.
 34. The recording method for a write-once opticalrecording medium according to claim 32, wherein after the laser beamhaving the power level P1 is irradiated, a laser beam having a powerlevel P2 higher than the read power Pr and lower than the write power Pwis further irradiated.
 35. The recording method for a write-once opticalrecording medium according to claim 32, wherein at least a length of ashortest recording mark among the recording marks is less than 0.35times the spot diameter of the laser beam.
 36. The recording method fora write-once optical recording medium according to claim 32, wherein atleast a width of the shortest recording mark among recording marks is0.7 times or more the length of the shortest mark.
 37. The recordingmethod for a write-once optical recording medium according to claim 32,wherein a wavelength of the laser beam is set within a range of 200 to450 nm.
 38. A write-once optical recording medium comprising a recordinglayer, the recording layer being to be irradiated with a laser beamhaving a predetermined recording strategy so as to record recordingmarks, wherein the recording strategy comprises a configuration suchthat, after a laser beam having a write power Pw is irradiated forforming a certain recording mark, another laser beam having a powerlevel P1 lower than a read power Pr set for reading recording marks isirradiated for a predetermined time.
 39. The write-once opticalrecording medium according to claim 38, wherein the recording layercomprises at least two sub recording layers each containing one kind ofmetal as its main component.
 40. The write-once optical recording mediumaccording to claim 39, wherein one of the sub recording layers containsas its main component metal one element selected from the groupcomprising Al, Ag, Au, and Cu.
 41. A write-once optical recording mediumaccording to claim 38, wherein the medium is to be irradiated with alaser beam having a wavelength of 450 nm or shorter at a recordingtransfer rate of 35 Mbps or higher, thereby forming the recording marks.42. The write-once optical recording medium according to claim 38,wherein at least a length of a shortest recording mark among therecording marks is less than 0.35 times the spot diameter of the laserbeam.